Generation of Clock Signal From Received Packet Stream

ABSTRACT

A receiving unit receiving packets of a packet stream having an evaluating unit for comparing the arrival times of the packets to a generated clock signal. The generated clock signal is used in transmitting information carried in the packets from the receiving unit. From the delays, a signal producing unit determines a characteristic delay value, which is characteristic of or typical of the statistical distribution of the delays. This characteristic delay value may be an average of the smallest delays of the distribution of delays for the latest received packets, and is used for adjusting the frequency of the generated clock signal. The average may be calculated or derived from all of the smallest delays of the distribution obtained for a predetermined share of the latest received packets.

FIELD OF THE INVENTION

The present invention relates to a receiving unit including devices forselecting packets to use for synchronization, to a method for generatinga clock signal and to a method of issuing an information stream from areceiving unit.

BACKGROUND

In the pending International patent application No. PCT/SE2005/00686,“Synchronization of VoDSL for DSLAM Connected Only to Ethernet”, filedMay 11, 2005, inventor Tore André, a method and a device forsynchronizing POTS (Plain Old Telephony Service) over packet networksare described, this previous patent application being incorporated byreference herein.

Generally, when packets are used for transporting informationrepresenting voice, a clock signal may be required, such as a samplingclock, when receiving the packets in order to be capable of forwardingthe voice information in another medium. The clock signal should berelated to the original clocking signal used in converting originalvoice information to a digital form and it may be used for convertingthe voice information carried in the packets to e.g. another digitalform, such as a continuous bit stream, or to an analog signal. Forexample, in a mobile telephony network the base stations may receivevoice information from a packet network, such as an Ethernet network,and then a packet clock signal may be regenerated in the base stationsto be used to create clock signals for transmitting information from thebase stations.

A telephone network including a packet transport network isschematically illustrated in FIG. 1 in which voice information iscommunicated through an Ethernet network. A telephone set 1 is throughan Ethernet interface 3 connected through an Ethernet networksymbolically illustrated by the Ethernet switch 5 to the PSTN (PublicSwitched Telephone Network) 7. The PSTN is connected to the Ethernetnetwork at a local exchange/switch 9 and a TAG (Telephony AccessGateway) 11. The TAG places the voice samples in Ethernet frames andretrieves voice samples from received Ethernet frames.

The Ethernet interface 3 is connected to the Ethernet network at anetwork port 13. The interface includes a plurality of line circuits 15,each connected to an individual subscriber line 17. The Ethernetinterface includes access equipment 19 that includes a module 21 forhandling packets received from and transmitted into the Ethernet and amodule 23 for handling calls, in particular for establishing andterminating calls and possibly for accounting.

Furthermore, the Ethernet interface 3 includes a clock device 25 takinginformation about incoming packets, such as from the network port 13,for generating a common clock signal distributed to the line circuits15. Ethernet frames of a single stream arriving to the Ethernetinterface can in an ideal case be supposed to arrive at equal distancesin time. However, due to delay variation in the transport network randomdelays are added to the expected arrival times. Such random delays canbe considered as a noise source in a timing recovery system, see FIG. 2.The noise is always positive but still a trend can be estimated, see thediagram of FIG. 3. The sloping line denoted by “Trend due to wrongclock” of FIG. 3 indicates in the case shown that the reference clocksignal used has a little too high a frequency. The line should in theideal case coincide with or be parallel to the abscissa axis of thediagram.

The clock device 25 can use, as described in the cited Internationalpatent application, an algorithm based on finding the smallest valuewithin a time window of arrival times of received packets. This methodis efficient if the distribution of delay values has a steep slope and alot of packets arrive with delays close to a minimum delay, minx, seethe diagram of FIG. 4.

In some cases, for e.g. a transport network having a high load, themajority of packets can be delayed more than the minimum delay, see thediagrams of FIGS. 5 a and 5 b. In those cases an algorithm using onlythe smallest values is not the best method since the packages arrivingwith very low delays are too rare.

SUMMARY

It is an object of the invention to provide a method and a receivingunit for producing a clock signal derived from arrival times of datapackets.

Generally, delays of arrived packets are obtained by comparing thearrival times of the packets to a generated clock signal. The delays areevaluated to form a characteristic delay value that is characteristic ofor typical of the statistical distribution of the delays. Thischaracteristic delay value is used for adjusting the frequency of thegenerated clock signal. The characteristic delay value may e.g. be anaverage of the smallest delays of the distribution, such as for thelatest received packets, or a value that may be said to represent, beindicative of or be substantially equivalent to such an average. Inparticular, the average or an equivalent quantity may be calculated orderived from all of the smallest delays of the distribution obtained fora predetermined share of the latest received packets, such as of packetsreceived during a time period ranging up to the current instant andhaving a predetermined length, or of a predetermined number of latestarrived packets.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of non-limiting embodimentswith reference to the accompanying drawings, in which:

FIG. 1 is a schematic of communicating voice information from/to atelephone set where a packet network using e.g. Ethernet is used fortransporting the voice information over some portion of thecommunication path to the public switched telephony network,

FIG. 2 is a diagram illustrating delays of packets received from apacket network,

FIG. 3 is a diagram illustrating a trend of delays in relation to clockreference signal for delays of packets received from a packet network,

FIG. 4 is a diagram illustrating a distribution of delays having a peakclose to a minimum delay value,

FIG. 5 a is a diagram similar to FIG. 4 for a distribution of delayshaving a peak at a distance of a minimum delay value,

FIG. 5 b is a diagram similar to FIG. 4 for a distribution of delayshaving two peaks,

FIG. 6 is a block diagram illustrating packets that are randomly delayedduring transmission from a source to a receiver,

FIG. 7 is a block diagram of a clock retriever unit in the receiver ofFIG. 6,

FIG. 8 a is a block diagram of a special unit in the retriever unit ofFIG. 7,

FIG. 8 b is a block diagram of an embodiment of the clock retriever unitof FIG. 7,

FIG. 9 is a block diagram of units for producing a characteristic delayvalue,

FIG. 10 is a block diagram of units required to produce a characteristicdelay value in another embodiment,

FIG. 11 is a block diagram of units required to produce a characteristicdelay similar to the units of FIG. 10.

FIG. 12 is a circuit diagram of units required to produce acharacteristic delay value according the block diagram of FIG. 11,

FIGS. 13 a-13 d are graphs illustrating delay distributions and theirmean and median values,

FIG. 14 is a block diagram of a unit for evaluating and producing acontrol signal,

FIG. 15 is a block diagram similar to FIG. 14 in which an evaluatingunit includes a delay evaluation unit,

FIGS. 16 a and 16 b are block diagrams of two embodiments of a delayevaluation unit,

FIG. 17 is a block diagram similar to FIG. 14 of a unit in which a delayevaluation unit includes units for determining characteristics of thedistribution of delays,

FIG. 18 is a block diagram similar to FIG. 11 including units formodifying an adaptation speed parameter, and

FIG. 19 is a diagram of a simple circuit for producing a characteristicdelay value.

DETAILED DESCRIPTION

In FIG. 6 is schematically illustrated how packets 31 are issued from apacket source 33 at periodically repeated times, as controlled by apacket clock unit 35 that e.g. provides a suitable clock signal. Thepackets are transported in some way, such as in wires or wirelessly,over a unit 37 introducing statistically varying delays, such as atransport network. The packets are from the delay introducing unitreceived by a packet receiver unit 39. A packet clock retrieving unit 41associated with the receiver unit uses the received packets to generatea clock signal having a frequency that as good as possible agrees withthe frequency with which the packets are issued from the packet source33. The regenerated clock signal is used in the receiver unit 39 forissuing information based on the contents of the received packets to adestination 43, this information also transported in some suitable way,such as in wires or wirelessly. In one case, the packet source 33receives voice information from e.g. a PSTN 7, see FIG. 1, converts itto digitally coded information and packetizes it, the clock used for thecoding having a frequency of 8 kHz and the clock used for transmittingthe produced packets being dependent on this 8 kHz clock. The packetstransporting the voice information are received by the receiver unit 39in which the 8 kHz frequency is restored (41) and used for converting,such as in the line circuits 15 of FIG. 1, in a digital-to-analogconverter, the information of the packets to an analog signaltransmitted to the destination 43. In another case, the regeneratedclock signal may be used for restoring a packet stream issued with samefrequency or period as the original packet stream to the destinationunit 43 or a bit stream having a frequency related to the frequency withwhich the packets are issued from the packet source 33. The delaysintroduced in the transport of the packets can have distributions asillustrated in the graphs of FIGS. 4, 5 a and 5 b in which, on theordinate direction, the number of packets is given for delays that areindicated on the abscissa axis. The delay distribution can for exampletypically be assumed to be varying rather slowly in time.

The clock retrieving unit 41, also called an adaptive clock generator,may have a structure e.g. as illustrated in the block diagram of FIG. 7.It includes a controllable or adjustable clock signal generator 45, anaccessing unit 47 for accessing the arrival times of received packetsand a determining unit 49. The determining unit determines the delay foreach received packet from the arrival time of the packet in relation tothe corresponding clock signal or clock pulse generated by the clockgenerator 45. An evaluation and signal producing unit 51 is connected tothe determining unit 49 and produces, based on the determined delays, asignal for adjusting the clock generator 45, i.e. for adjusting thefrequency thereof. The unit 49 for determining delays of packets, theunit 51 for evaluating delays of packets and for producing a signal forcontrolling the controllable clock signal generator and the controllableclock signal generator 45 are in this embodiment components of a generalclock unit 52.

The evaluation and signal producing unit 51 can basically be built asillustrated in FIG. 8 a. Thus, it can include an evaluation unit 53 thatevaluates the delays of received packets and produces a characteristicdelay value that is derived from the delays and in some way is typicalof or characteristic of the delays. The characteristic delay value isprovided to a signal producing unit 55, also called a feedback unit or afilter unit, that from the received value produces a signal suitable forcontrolling the clock generator 45. The unit 55 may e.g. be a loopfilter of as typically included in a PLL (Phase Locked Loop).

A general clock unit 52 based on such a phase locked loop may includemain components as illustrated in the block diagram of FIG. 8 b. Theclock generator 45 is here a VCXO (Voltage controlled crystaloscillator) 45 and it receives its control signal from a D/A converter(DAC) 57. The fact that a packet has been determined to have beenarrived, as is determined in a unit 58, produces a signal directlycontrolling an n-bit register 59. When the n-bit register receives sucha control signal indicating that a new packet has just arrived, the fullcontent of an n-bit counter 60 is copied to the n-bit register. Then then-bit register 59 transfers its current content to a unit 53 forselecting packets. The transferred values are evaluated in the packetselecting unit and only the selected values are provided to a PLL loopfilter 61 that is a low-pass filter and generates a filtered value inputto the D/A converter 57. The regenerated clock signal obtained from theVCXO 45 is fed back to the n-bit counter 60 connected to the n-bitregister 59. The n-bit counter increments its value for each clock pulsegenerated, e.g. for each leading edge. The number of bits in the n-bitcounter 60 is selected, considering the frequency of pulses generated bythe VCXO, so that it wraps around every packet period.

For a distribution of delay values such as that of FIG. 4 having a steepslope and a lot of packets arriving with delays close to a minimumdelay, minx, an algorithm based on the arrival times of those receivedpackets which have the smallest delay values within a time window forthe arrival times of received packets can be used for determining thecharacteristic delay value.

For a distribution of delay values such as that of FIG. 5 a, that can beobtained for e.g. a transport network having a high load, the majorityof packets are delayed significantly more than the minimum delay. Then,the method of using packets having delays equal to or very close to theminimum delay is not efficient for determining a characteristic delayvalue. If e.g. a method using the average of the 20% lowest values insome window is used to find an estimate of the frequency or period withwhich the packets are issued can be derived, this estimate is probablybetter than that determined using the first discussed method and hencesuch a method is probably also more efficient.

Such a method involves the two steps:

Find, among the packets within the window, e.g. within the N packetswhich have most recently arrived or within a time window, i.e. thepackets which have arrived within the time period of length T from thecurrent time, where N is a fixed number and T is a fixed time length,the packets of interest, e.g. the p·N packets that have the smallestdelays, where p is the considered share, e.g. equal to 20%.

Calculate the average of the delays of the found packets or find someequivalent quantity, value or signal, such as by a low-pass filteringoperation, suitable for controlling the clock generator 45.

However, there is a problem to implement the first of these steps usingan acceptable, not too high, number of calculations. To find the packetsthat have the lowest delay values within e.g. a time-window normallyrequires a sorting procedure, see the block diagram of FIG. 9. In thisfigure two units are illustrated, that could be included in theevaluation and signal producing unit 51, a sorting and selection unit65, that may be considered to correspond to the evaluating unit 53, forperforming the operations necessary to select the 20% lowest delayvalues and an average calculation unit 67 for determining the average ofthe selected delay values or some equivalent quantity as describedabove. The sorting operation is a complex procedure that in turnrequires a lot of operations, in particular a multitude of comparisons.

Other methods can be used, e.g. the method illustrated in the blockdiagram of FIG. 10, in which the delay values are compared to a variablethreshold delay value corresponding to the 20% limit. Values in thewindow that fall under the threshold T_(p) are used in the averagecalculation. The threshold delay value should be adaptive, constantlyand automatically adapting to the 20% limit, this implying that thenumber of packets having delay values smaller than the threshold T_(p)may not be exactly equal to the selected share, e.g. 20%, of the packetswithin the window. The value 20% is only an example and higher and lowervalues can be used, if desired or suitable, as will be discussedhereinafter. In FIG. 10 units that may be necessary for performing thismethod are illustrated and that may be included in the evaluation andsignal producing unit 51. A comparing unit 69 compares the determineddelay of each received packet to the threshold T_(p) and provides thedelay value only if higher than the threshold to an averaging unit 67for determining the average of only the selected delay values within asliding window. A threshold adaptation unit 71 is arranged to adapt thethreshold T_(p) used in the comparing operation of the comparing unitand may for the adapting operation use the result of the comparing ordirectly the delays of received packets.

In the operation of adapting the threshold T_(p) the numbers of packetshaving delay values larger and smaller than the threshold are counted.The threshold value is adjusted in such a way that the numbers ofpackets having values over and under the threshold correspond to thepreferred percentage of e.g. 20%. This may be done by adjusting thethreshold T_(p) according to the following algorithm where x is thedelay value of a received packet in relation to the clock signalcurrently generated by the clock retriever unit 41:

For each received packet having a delay value under or equal to thethreshold, the threshold T_(p) is decreased.

For each received packet having a delay value over the threshold, thethreshold T_(p) is increased.

If the increase of the threshold T_(p) is proportional to the desiredprobability for an approved value p, i.e. p=0.2 for 20% in the example,and the reduction of the threshold is equal to (1−p), i.e. equal to 0.8in the example, the algorithm will adapt the threshold to the correctvalue. A small number μ, e.g. equal to 0.01, determines the adaptationspeed. The procedure can be described by the following pseudocodesegment, see also FIG. 11:

DO for all received values x IF x < threshold threshold = threshold −μ·(1−p) use x in the average calculation ELSE threshold = threshold +μ·p do not use x in the average calculation END END

The procedure may be performed by the units illustrated in the blockdiagram of FIG. 11, these units e.g. included in the evaluation andsignal producing unit 51 and in particular in the evaluating unit/packetselecting unit 53. Due to the result of the comparison in the comparingunit 69, one of two parallel adaptation units 71′ and 71″ makes amodification of the threshold value T_(p), according to the respectiveformula above. In the same way, one of two units 73, 75 sets the use ofthe delay value in the averaging operation performed by theaveraging/filtering unit 67.

The functions of the two adaptation units 71′ and 71″ may be performedby a single adaptation unit 77. An example of the internal structure ofsuch a single adaptation unit appears from the diagram of FIG. 12. Herethe comparing unit 69 is a comparator e.g. outputting a signal havingthe value 0 or 1 depending on the result of the comparing operation.This signal is used to control a switch 79 connected to fixed signalsrepresenting the values p and (p−1), respectively, one of these signalsthus provided to a multiplying circuit or amplifying circuit 81,amplifying the selected signal by a factor representing the quantity μdefining the adaptation speed, and providing the amplified signal to asumming or integrating unit 83 including a hold circuit 85 and an adder87, the adder also receiving as input a value representing the currentthreshold value held by the hold circuit. From the output of the summingunit that is the same as the output of the hold circuit, the signalrepresenting the current threshold value is provided to the comparator69.

In the case where the x value is not to be used because it is above thethreshold T_(p), preferably zero can be inserted of this delay value. Analternative is to only input the delay values x that are lower than orequal to the threshold T_(p), i.e. not to insert zeroes. In the lattercase the average can be taken over a certain number of approved valuesrather than a certain time window or window of received packets. Asillustrated in the diagram of FIG. 12, this can be achieved by havingthe output signal of the comparator 69 control a second switch 89 thatprovides either the signal representing the delay of the most recentlyreceived packet to the averaging unit 67 or a signal representing zeroto that unit.

The value of the threshold T_(p) is only used for finding the delaysfrom which the desired average is determined. However, this value alsocontains information on the shape of the distribution of the delays.Hence, for instance the threshold value T_(p) for p=50% is the median ofthe delay distribution. Information on the shape of the currentdistribution can be used for controlling the value of the parameter pactually used for the averaging operation and hence for controlling thefrequency of the clock signal generated by the clock generator 45. Fore.g. the different delays distributions illustrated in FIGS. 4, 5 a and5 b different suitable p-values can be used and these distributions canbe distinguished from each by characteristic values that can be easilycalculated.

The graphs of FIGS. 13 a-13 d show typical delay distributions. The meanvalue is indicated by a vertical line and the median value by anasterisk. Depending on the mean and median values different percentvalues of p can be chosen.

1. FIG. 13 a illustrates a distribution having a very long andsubstantial trail extending up to high delays. The median value is muchsmaller than the mean value. The value of p can suitably be selected toe.g. 60%.

2. FIG. 13 b illustrates a distribution having a pronounced double peakstructure. The median value is larger than the mean value. The value ofp can suitably be selected to e.g. 30%.

3. FIG. 13 c illustrates a symmetric delay distribution having a singlepeak. The median value is substantially equal to the mean value. Thevalue of p can suitably be selected to e.g. 100%.

4. FIG. 13 c illustrates a delay distribution having a high peak at alow delay values and a moderately large trail extending to not too highvalues. The median value is somewhat smaller than the mean value. Thevalue of p can suitably be selected to e.g. 60%.

In order to provide a modifiable p-value, the unit 51 for evaluatingdelays of packets and producing a signal for controlling the clockgenerator 45 has to be supplemented with some components. As seen in theblock diagram of FIG. 14, the unit 51 then may as above contain theevaluating unit 53 that in turn includes the comparator 69 for comparingdelays of received packets to the current value T_(p) of the adaptivethreshold for the share given by the parameter p. The value of theparameter p is for each received packet modified by a modifying unit 91also included in the evaluating unit 53 that in addition includes a unit93 for modifying the value of the parameter p. The unit 93 can e.g.examine the delays for received packets and make some choice based onthe result of the examination to find a modified value of the parameterp. The change of the parameter p should be made rather slowly,significantly slower than changing the signal output from low-passfilter 55′, corresponding to the signal producing unit/filtering unit 55of FIG. 8 a. Thus, the characteristic time τ_(p) for changing the valueof the parameter p should be long compared to the characteristic timeτ_(d) for changing the control signal provided to the clock generator.

The modifying unit 93 may as seen in FIG. 15 include a unit 95 forevaluation of the delays, a unit 97 for calculation of a new value ofthe parameter p and a unit 99 for delaying the change of this value, theunit 99 for example being a low-pass filter. The evaluation unit 95 canbe arranged to in some way determine the shape of the distribution ofthe delays of the latest received packets. Such determining can be madein a unit 101 outputting the result of the determining to a unit 103 forevaluating the determined distribution, see FIG. 16 a. Moreparticularly, as seen in FIG. 16 b, first some characteristics of thedistribution of the latest determined delays can be determined in a unit101′ and then these determined characteristics evaluated in a unit 103′.This evaluation unit 103′ can e.g. be arranged to make the evaluationaccording to a suitable algorithm or by a table look-up, the algorithmor table determined by e.g. simulating packet traffic.

Then, the characteristics determining unit 101′ may include a unit 105for determining the median, see FIG. 17. The median determining unit isbuilt from a comparator 107 connected to receive determined delays andconstructed similar to the comparator 69 but set for a constant value50%. It is connected to a threshold modifying unit 109 that is alsoincluded in the median determining unit and is similar to the thresholdmodifying unit 91. The determined threshold value may be stored, ifnecessary, in a memory cell 110. Furthermore, the characteristicsdetermining unit 101′ can include a unit 111 for determining the averageof the delays of the latest received packets. The average determiningunit may, if required store the determined average value, also calledmean value, in a memory cell 112. The average determining 111 unit canbe designed, as has above been discussed for averaging units 55, 55′ and67, as a low-pass filter and it provides the determined average to theevaluation unit 103′, that also receives the determined median valuefrom the modifying unit 109. The evaluation unit may use a table storedin a memory place 104 to find, for each combination av median and meanvalues, a suitable p-value.

In order to increase the speed of the threshold adaptation performed inthe units 71′, 71″; 91 and 109 a variable step algorithm can be used.The principle idea is that if the threshold T_(p) is very far from itsoptimum value, either too high or to low, the delay values will mainlybe below or above the threshold. An algorithm may be based on theprinciple that if adjacent values are of the same character, i.e. bothabove or both below the threshold, the speed of adaptation is increasedby increasing the value of the adaptation speed parameter μ. If adjacentvalues are not of the same character, i.e. one above and one below thethreshold, the speed of adaptation is decreased.

If the threshold T_(p) is placed at e.g. 10%, i.e. p=0.1, hence at 10%of the values below the threshold, then it is more likely to get severalvalues above the threshold. To compensate for this, the step isincreased less if the values are above the threshold T_(p) than if thevalues are above the threshold. The procedure can be described by thefollowing pseudocode segment:

DO for all received values x IF x < threshold IF last_value = ‘below’ μ= μ·(1 + (1 − p)·0.005) ELSE μ = μ/(1 + (1 − p) ·0.005) END last value =‘below’ . . . ELSE IF last_value = ‘above’ μ =μ·(1 + p·0.005) ELSE μ =μ/(1 + p·0.005) END last value =‘above’ END END

The lines including a division operation can be replaced with

μ=μ·(1−(1−p)·0.005)

μ=μ·(1−p·0.005)

The value 0.005 is here generally some small, suitably selected numberthat characterizes the speed with which the adaptation speed of thethreshold changes. The algorithm can be easily performed in a unitincluded in the evaluating unit 53 and working in parallel with theunits 69, 71′ and 71″ of FIG. 11 and the units 69, 91 and 93 of FIGS.14, 15 and 17. Hence, as seen in FIG. 18, determined values x of thedelays of received packets are as above in the comparator 69 compared tothe current value of the threshold T_(p). If a delay value is smallerthan the threshold, it is determined in a unit 117 whether the valuecalled “last value” stored in a memory cell 118 is equal to thecharacter string ‘below’. If it is true the value of threshold speedadaptation parameter μ is modified in a unit 119′. Otherwise it ismodified in a parallel unit 119″. Finally, the value called “last value”is in a unit 121 set to the character string ‘below’ and stored in thememory cell 118. If it is determined in the comparator 69 that the valuex is not smaller than the threshold, it is similarly tested in a unit123 whether the stored value called “last value” is equal to thecharacter string ‘above’. If it is true, the parameter μ is modified ina unit 125′ and else in a parallel unit 125″ In the latter two cases,the value called “last value” is in a unit 127 set to the characterstring ‘above’ and stored in the memory cell 118. The evaluating unit 53generally includes or is connected to a memory 129 in which the memorycell 118 can be a part and which may include memory cells 131, 133, 135for storing the current value of the parameter μ, the current value ofthe share parameter p and the current value of the threshold Tp.

The unit 67 for calculating a sliding window average may, as has beendescribed above, be designed as a low-pass filter, e.g. incorporated inor constituting the signal producing/filtering unit 55. The feedbackloop of such a filter should generally have a much larger time constantthan the changes of the adaptation threshold T_(p), i.e. the delaybefore sending a signal to change the generated frequency is much longerthan the time periods elapsed before a changed adaptation threshold isdetermined.

Alternatively, the operation of calculating a sliding window average canbe implemented as a separate unit that e.g. as seen in FIG. 19, in asimple case, consists of a delay unit 141, delaying the provision ofdelay values by N steps, and an integrator 143 including a hold circuitconnected in series.

While specific embodiments of the invention have been illustrated anddescribed herein, it is realized that numerous other embodiments may beenvisaged and that numerous additional advantages, modifications andchanges will readily occur to those skilled in the art without departingfrom the spirit and scope of the invention. Therefore, the invention inits broader aspects is not limited to the specific details,representative devices and illustrated examples shown and describedherein. Accordingly, various modifications may be made without departingfrom the spirit or scope of the general inventive concept as defined bythe appended claims and their equivalents. It is therefore to beunderstood that the appended claims are intended to cover all suchmodifications and changes as fall within a true spirit and scope of theinvention. Numerous other embodiments may be envisaged without departingfrom the spirit and scope of the invention.

1. A receiving unit receiving packets including a device for producing,from received packets, a clock signal, the device including: a clocksignal generator for generating a clock signal having a controllablefrequency, a delay determining unit determining the delays of receivedpackets in relation to the clock signal, the delays of most recentlyreceived packets forming a delay distribution ranging from a currentminimum delay value to larger delay values, and an evaluating andcontrol signal producing unit for evaluating delays and for producing acontrol signal provided to the clock signal generator based on thedelays, the evaluating and control signal producing unit including meansfor determining a characteristic delay value, which is characteristic ofthe delay distribution and for producing the control signal from thecharacteristic delay value.
 2. The receiving unit according to claim 1,wherein the means for determining the characteristic delay valuedetermines the characteristic delay value as an average of the smallestdelays of the delay distribution.
 3. The receiving unit receivingaccording to claim 1, wherein the means for determining thecharacteristic delay value determines the characteristic delay value asan average of all of the smallest delays of the distribution obtainedfor a predetermined share of received packets.
 4. The receiving unitaccording to claim 3, wherein the evaluating and control signalproducing unit includes an evaluating unit including: a memory or memorycell for holding a variable threshold value, a comparing unit forcomparing the determined delay of a received packet to the thresholdvalue, an adaptation unit connected to the comparing unit for adjustingthe threshold value dependent on the result of the comparing, and anaveraging unit or low pass filter connected to the comparing unit fordetermining an average or filtered value of delays of selected ones ofreceived packets, the selecting being dependent on the result of thecomparing.
 5. The receiving unit according to claim 4, wherein theevaluating unit includes: a memory or memory cell for holding a variableadaptation speed value, wherein the adaptation unit also adjusts thethreshold value dependent on the adaptation speed value.
 6. A method ofproducing a clock signal in a receiving unit receiving packets, saidmethod comprising the steps of: generating a clock signal having acontrollable frequency, determining delays of received packets inrelation to the clock signal, the delays of most recently receivedpackets forming a delay distribution ranging from a current minimumdelay value to larger delay values, and evaluating delays and producinga control signal for controlling the frequency of the clock signal basedon the determined delays, by forming a characteristic delay value, whichis characteristic of the delay distribution and producing the controlsignal from the formed characteristic delay value.
 7. The methodaccording to claim 6, wherein in forming the characteristic delay value,the characteristic delay value is formed as an average of the smallestdelays of the delay distribution.
 8. The method according to claim 6,wherein in forming the characteristic delay value, the characteristicdelay value is formed as an average of all of the smallest delays of thedelay distribution for a predetermined share of received packets.
 9. Themethod according to claim 8, wherein in forming the average thefollowing steps are executed: determining, for each received packet, thedelay of the packet, comparing the determined delay of each receivedpacket to a variable threshold value, adjusting the variable thresholdvalue dependent on the result of the comparing, selecting determineddelays dependent on the result of the comparing, and determining anaverage of the selected determined delays or a filtered value from theselected determined delays.
 10. The method according to claim 9, whereinthe step of adjusting the variable threshold value includes adjustingthe variable threshold value dependent on a variable adaptation speedvalue.
 11. A method of issuing information from a receiving unitreceiving packets, said method comprising the steps of: generating aclock signal having a controllable frequency, determining delays ofreceived packets in relation to the clock signal, the delays of mostrecently received packets forming a delay distribution ranging from acurrent minimum delay value to larger delay values, evaluating delaysand producing a control signal for controlling the frequency of theclock signal based on the determined delays, by forming a characteristicdelay value characteristic of the delay distribution and producing thecontrol signal from the formed characteristic delay value, and issuinginformation from the receiving unit based on the generated clock signal.12. The method according to claim 11, wherein the characteristic delayvalue is formed as a quantity or signal substantially equivalent to anaverage of the smallest delays of the delay distribution.
 13. The methodaccording to claim 11, wherein the characteristic delay value is formedas a quantity or signal substantially equivalent to an average of all ofthe smallest delays of the delay distribution for a predetermined shareof received packets.
 14. The method according to claim 13, wherein informing the quantity or signal substantially equivalent to said averagethe following steps are executed: determining, for each received packet,the delay of the packet, comparing the determined delay of each receivedpacket to a variable threshold value, adjusting the variable thresholdvalue dependent on the result of the comparing, selecting determineddelays dependent on the result of the comparing, and determining anaverage of the selected determined delays or a filtered value from theselected determined delays.
 15. The method according to claim 14, inwherein the step of adjusting the variable threshold value includesadjusting the variable threshold value dependent on a variableadaptation speed value.